Method for making and using a drilling fluid

ABSTRACT

A drilling fluid and method for drilling in a coal containing formation. The method includes: providing a mixed metal-viscosified drilling fluid including at least 1% potassium salt; circulating the drilling fluid through the well; and drilling into a coal seam.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 13/776,914 filed Feb. 26, 2013. U.S. Ser. No. 13/776,914 is a divisional of U.S. application Ser. No. 12/529,583 filed Dec. 22, 2009, now U.S. Pat. No. 8,387,723 issued Mar. 5, 2013, which is a 371 of PCT/CA2008/00425 filed Mar. 3, 2008, which claims benefit of U.S. 60/892,672 filed Mar. 2, 2007.

FIELD

This invention relates to methods and fluids used for drilling and completing oil wells.

BACKGROUND

The process of drilling a hole in the ground for the extraction of a natural resource requires a fluid for removing the cuttings from the wellbore, lubricating and cooling the drill bit, controlling formation pressures and maintaining hole stability.

Many earth formations contain coal seams through which a wellbore must be drilled to either access the coal itself or reservoirs of interest below the coal.

For coal bed methane (CBM) wells, minimization of formation damage is paramount given the lower permeability of coal seams than conventional reservoirs. A fluid that minimizes the formations damage and reduces whole mud loss by limiting the invasion into the cleats and fractures and permits easy flow back has been developed, termed herein the mixed metal-viscosified drilling fluids including mixed metal oxide (MMO), mixed metal hydroxide (MMH) and combinations of mixed metal oxide and hydroxide (MMOH). The mixed metal-viscosified drilling fluids contain a mixed metal viscosifier, which is an inorganic particle based on magnesium/aluminum oxides and/or hydroxides. The mixed metal particles have a cationic character and react electrostatically with clay particles. Mixed metal-viscosified drilling fluids include an aqueous-based mixture of at least one of the mixed metal moieties and an amount of bentonite. The rheology of mixed metal-viscosified drilling fluids limits fluid invasion into the formation due to high viscosity but the main formation protection comes from the formation of an external filter cake that is easy to remove. Simple displacement to water or brine should be sufficient for the well to flow back and remove the filter cake.

Unfortunately, however, the rheology of mixed metal-viscosified drilling fluids has broken down when coming into contact with coal fines generated from drilling into coal seams, especially young coal. When the drilling fluid comes in contact with coal fines generated by drilling through the seams, the fluid thins, moving toward the rheology of water and therefore loses many of its beneficial properties. Since coal seams are, in fact, often considered loss zone formations, and are weak and friable, the unsuitability of mixed metal-viscosified drilling fluids for drilling in coal containing formations is particularly problematic.

SUMMARY OF THE INVENTION

In accordance with a broad aspect of the present invention, there is provided a method for drilling in a coal containing formation, the method comprising: providing a mixed metal-viscosified drilling fluid including at least 1% potassium salt; circulating the drilling fluid through the well; and drilling into the coal seam.

In accordance with another broad aspect of the present invention, there is provided a drilling fluid comprising: an aqueous mixture of bentonite and a mixed metal viscosifier with a pH above about pH 10; and at least 1% potassium salt.

It is to be understood that other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein various embodiments of the invention are shown and described by way of example. As will be realized, the invention is capable for other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly the detailed description and examples are to be regarded as illustrative in nature and not as restrictive.

DESCRIPTION OF VARIOUS EMBODIMENTS

The detailed description and examples set forth below are intended as a description of various embodiments of the present invention and are not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for the purpose of providing a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

Until now mixed metal (MMO, MMH and MMOH) viscosified drilling fluids have been used generally unsuccessfully in coal seams due to the fluid thinning effect from the coal. It is believed that the polyanionic nature of coal fines, such as of lignite and lignosulfonates, interfere with the electrostatic interactions of the mixed metal moiety and the bentonite in the drilling fluid, sometimes resulting in a complete collapse of the fluid's rheology.

We have determined that some salts reduce or prevent the thinning effect from drilling coals with MMO, MMH and MMOH viscosified fluids. Potassium salts including one or more of potassium sulfate, potassium chloride, potassium acetate and potassium formate may substantially maintain the rheology of mixed metal-viscosified drilling fluids when drilling with coal contaminants. Such salts may add a benefit of shale swelling inhibition, possibly as a result of the presence of the potassium ion from the salt.

A wide range of potassium salt concentrations, such as concentrations greater than 1% (weight by volume), may be effective in the mixed metal-viscosified drilling fluid. Generally concentrations of 1-10% (weight by volume) salt and, for example, 1-5% salt (weight by volume) concentrations have been found to be both effective for stabilizing the drilling fluid against adverse rheological changes due to coal contamination and advantageous in terms of economics. The amount of salt added to the drilling fluid may be determined by the amount of coal to be drilled and/or by the shale reactivity. For example, younger coals, more so than older coals, tend to create greater rheological instability for mixed metal-viscosified drilling fluids and, thus, higher concentrations (for example greater than 3% and for example 3-10%) of potassium salts in the drilling fluid may be useful. Also, if it is determined that there are significant coal deposits through which the well must be drilled, again higher concentrations of potassium salts may be useful.

Although the salt may be added after the coal contamination occurs, it is recommended to pre-treat the system for best results. In one embodiment, for example, the surface hole can be drilled down to approximately the level of the first coal deposit using any drilling fluid of interest, including for example, prior art mixed metal-viscosified drilling fluids. When it is determined that the coal seam is close below bottom hole or when the coal seam has been reached, the drilling fluid may be changed over to a drilling fluid according to the present invention, including a mixed metal-viscosified drilling fluid containing an amount of a potassium salt.

Alternately, the borehole may be drilled down to and through a coal seam using a drilling fluid according to the present invention. For example, the entire well substantially from surface, which it will be appreciated may include drilling from surface or from below the overburden or after the casing point, may be drilled using a drilling fluid according to the present invention.

After drilling through the coal seams in the path of the borehole, the present drilling fluid may continue to be used for the remainder of the wellbore or other drilling fluids may be used. However, if coal fines may continue to become entrained in the drilling fluid, for example where a coal seam remains open to contact by the drilling fluid, it may be useful to continue using the present drilling fluid until drilling is complete or the possibility of coal contamination is eliminated. If desired, the drilling fluid returning to the mud tanks at surface may be monitored to determine the concentration of potassium salt therein, as well as other parameters, to ensure that appropriate levels and fluid characteristics are maintained. For example, any one or more of the bentonite, mixed metal viscosifier, base, or potassium salt may be added during drilling to adjust the drilling fluid parameters. In one embodiment, for example, an amount of mixed metal viscosifier may be added to the fluid during the course of a drilling operation where reactive formations are drilled and drill cuttings become incorporated to and change the rheology of the drilling fluid. In such a case, the addition of an amount of mixed metal viscosifier can cause the viscosity of the fluid to increase.

As will be appreciated, the drilling fluid may be circulated through the drill string, drill bit and well bore annulus while drilling. Circulation of the drilling fluid may continue even when drilling is stopped in order to condition the well, prevent string sticking, etc.

During the drilling and circulation, the yield point of the drilling fluid may be maintained above 10 Pa to provide advantageous effects.

Mixed metal-viscosified drilling fluids include bentonite and a mixed metal viscosifier in water and are pH controlled.

Bentonite is commonly used in drilling fluids and its use will be well understood by those skilled in the art. While various forms of bentonite may be used, bentonites that contain polyanionic additives or impurities should be avoided, with consideration as to the electrostatic interaction of the bentonite and MMOH. An untreated bentonite may be particularly useful. Such a bentonite may be known commercially as untreated bentonite with a high content of sodium montmorillonite or untreated Wyoming bentonite.

Mixed metal viscosifiers are commercially available such as from BASF Oilfield Polymers Inc. under the trademark Polyvis™.

Generally, mixed metal-viscosified drilling fluids may include low concentrations of bentonite (for example, about 15 to 45 kg/m3 or 20 to 40 kg/m3 bentonite in water). Considering that many bentonite based (non-mixed metal) drilling fluids can contain many multiples more (i.e. two to four times) bentonite than in a mixed metal-viscosified drilling fluid, it can be appreciated that the viscosity generated using such low concentrations of bentonite for mixed metal-viscosified drilling fluids might be insufficient for hole cleaning. The addition of mixed metal oxide, mixed metal hydroxide or mixed metal oxide and hydroxide at a weight ratio of 1:8 to 1:12 or 1:9.5 to 1:10.5 to the bentonite produces a stable fluid when the pH is initially maintained above about 10.0 and possibly between about 10.5 and 13, as may be achieved by addition of caustic soda, caustic potash, potassium carbonate and/or soda ash. Once the bentonite/mixed metal viscosifier reaction is complete and a gel is formed, it appears that the pH can be lowered to pH 9 or possibly even lower without any significant loss in viscosity.

In one embodiment, a mixed metal-viscosified drilling fluid may include an aqueous mixture of about 30 kg/m3 bentonite, a mixed metal moiety in a quantity of about 1:10 MMO, MMH or MMOH to bentonite, pH controlled to greater than pH 11 and 1 to 5% potassium salt.

Additives for fluid loss control, lost circulation, etc. may be added to the drilling fluid mixture, as desired. Non or minor-ionic additives may be most useful. Some examples may include starch for fluid loss reduction, organophillic lost circulation materials (LCM), etc. Simple testing may verify the compatibility of any particular additive with the drilling fluid.

To produce the drilling fluid, the bentonite may first be hydrated in water. Then the mixed metal moiety is added and pH is adjusted. The potassium salt can be added to the aqueous mixture of bentonite and mixed metal any time before it is needed for drilling with coal contamination. Additives such as LCM, fluid loss control agents, etc. can also be added when appropriate, as will be appreciated.

A typical drilling fluid formulation may be according to Table 1.

TABLE 1 A typical drilling fluid useful for drilling in coal-containing formations Product Concentration Notes Untreated bentonite 30 kg/m3 Prehydrate first in fresh water MMH or MMO or MMOH 3 kg/m3 Caustic Soda 0.5 to 1 kg/m3 To control pH at 11-12.5 Potassium Sulfate 20 to 50 kg/m3 Starch 5 to 10 kg/m3

The following examples are included for the purposes of illustration only, and are not intended to limit the scope of the invention or claims.

EXAMPLES Example I

In the following examples, drilling fluids were prepared according to the sample descriptions by hydrating the bentonite, adding the mixed metal moiety and adjusting the pH, as needed. Thereafter, any additives, including potassium salt if any, were added.

To simulate coal contamination, lignite was added.

The rheological properties have been tested using a Fann 35 and Brookfield viscometers.

TABLE 2 Composition of Sample #1 Products Sample #1 Untreated Bentonite 30 kg/m3 MMH  3 kg/m3 Caustic 0.5 kg/m3  Starch 10 kg/m3

TABLE 3 Results without the addition of Salt Sample #1 + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties Sample #1 Lignite Lignite 600 RPM 86 47 43 300 RPM 64 29 25 200 RPM 53 21 18 100 RPM 40 13 10  6 RPM 19 2 1.5  3 RPM 17 1 1  10 sec Gel (Pa) 8 1 0.5 PV (mPa * s) 22 18 18 YP (Pa) 21 5.5 9 LSRV (cP) 54,000 12,000 0 Temperature (° C.) 22.8 22.3 23.0

TABLE 4 Results using Potassium Chloride Sample #1 + Sample #1 + 2% KCl + 2% KCl + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties 2% KCl Lignite Lignite 600 RPM 66 47 44 300 RPM 52 31 27 200 RPM 46 23 21 100 RPM 38 16 14  6 RPM 18 4 3  3 RPM 16 3 2  10 sec Gel (Pa) 7 2 1.5 PV (mPa * s) 14 16 17 YP (Pa) 19 7.5 5 LSRV (cP) 25,000 12,000 9,000 Temperature (° C.) 21.6 22.1 22.3

TABLE 5 Results using Potassium Acetate Sample #1 + Sample #1 + 2% Pot. Acetate + 2% Pot. Acetate + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties 2% Pot. Acetate Lignite Lignite 600 RPM 66 52 48 300 RPM 47 38 35 200 RPM 39 32 29 100 RPM 30 25 22  6 RPM 12 10 10  3 RPM 8 8 7  10 sec Gel (Pa) 4 4 4 PV (mPa * s) 13 14 13 YP (Pa) 20 12 11 LSRV (cP) 31,000 20,000 12,000 Temperature 23.2 23.3 23.2 (° C.) Note: Lignite dissolves slower.

TABLE 6 Results using Potassium Formate Sample #1 + Sample #1 + Sample #1 + 2% Pot. Formate + 2% Pot. Formate + 2% Pot. 5 kg/m3 15 kg/m3 Mud Properties Formate Lignite Lignite 600 RPM 66 47 42 300 RPM 53 32 28 200 RPM 47 26 22 100 RPM 38 18 16  6 RPM 19 6 5  3 RPM 18 4 4  10 sec Gel (Pa) 7 2 2 PV (mPa * s) 13 15 14 YP (Pa) 20 8.5 7 LSRV (cP) 21,000 13,000 12,000 Temperature 22.1 22.3 22.6 (° C.)

TABLE 7 Results using Calcium Nitrate Sample #1 + 2% Sample #1 + Sample #1 + Calcium Nitrate + 2% Calcium Nitrate + 2% Calcium 5 kg/m3 15 kg/m3 Mud Properties Nitrate Lignite Lignite 600 RPM 60 57 47 300 RPM 46 42 34 200 RPM 38 34 28 100 RPM 31 27 22  6 RPM 12 11 7  3 RPM 9 9 5  10 sec Gel (Pa) 5 5 3 PV (mPa * s) 14 15 13 YP (Pa) 16 13.5 10.5 LSRV (cP) 33,000 23,000 22,000 Temperature 21.5 22.1 22.7 (° C.) Note: Lignite dissolves slower.

TABLE 8 Results using Calcium Chloride Sample #1 + Sample #1 + 2% Calcium 2% Calcium Sample #1 + Chloride + Chloride + 2% Calcium 5 kg/m3 15 kg/m3 Mud Properties Chloride Lignite Lignite 600 RPM 61 51 47 300 RPM 44 35 34 200 RPM 36 30 29 100 RPM 27 22 23  6 RPM 10 8 8  3 RPM 8 7 6  10 sec Gel (Pa) 3.5 3.5 3 PV (mPa * s) 17 16 13 YP (Pa) 13.5 9.5 10.5 LSRV (cP) 27,000 23,000 22,000 Temperature (° C.) 24.4 24.4 24.2 Note: Lignite dissolves slower.

TABLE 9 Results using Potassium Sulfate Sample #1 + Sample #1 + Sample #1 + 2% Pot. Sulfate + 2% Pot. Sulfate + 2% 5 kg/m3 15 kg/m3 Mud Properties Pot. Sulfate Lignite Lignite 600 RPM 75 42 34 300 RPM 60 29 21 200 RPM 52 24 16 100 RPM 41 18 11  6 RPM 21 8 2.5  3 RPM 19 7 2  10 sec Gel (Pa) 9 4 2.5 PV (mPa * s) 15 13 13 YP (Pa) 22.5 8 4 LSRV (cP) 32,000 30,000 25,000 Temperature (° C.) 24.4 24.0 21.3

TABLE 10 Results using Potassium Chloride Sample #1 + Sample #1 + 5% KCl + 5% KCl + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties 5% KCl Lignite Lignite 600 RPM 61 52 46 300 RPM 49 39 35 200 RPM 45 35 32 100 RPM 42 32 30  6 RPM 16 15 15  3 RPM 12 11 10  10 sec Gel (Pa) 6 6 5 PV (mPa * s) 12 13 11 YP (Pa) 18.5 13 12 LSRV (cP) 30,000 18,000 21,000 Temperature (° C.) 20.1 20.1 20.1

TABLE 11 Results using Potassium Acetate Sample #1 + Sample #1 + 5% Pot. Acetate + 5% Pot. Acetate + Sample #1 + 5 kg/m3 15 kg/m3 Mud Properties 5% Pot. Acetate Lignite Lignite 600 RPM 63 48 44 300 RPM 55 37 36 200 RPM 51 36 34 100 RPM 47 34 32  6 RPM 14 20 16  3 RPM 9 11 11  10 sec Gel (Pa) 5 5 6 PV (mPa * s) 8 11 8 YP (Pa) 23.5 13 14 LSRV (cP) 27,000 14,000 33,000 Temperature 20.1 20.1 20.1 (° C.) Note: Lignite dissolves slower.

TABLE 12 Results using Potassium Formate Sample #1 + Sample #1 + Sample #1 + 5% Pot. Formate + 5% Pot. Formate + 5% 5 kg/m3 15 kg/m3 Mud Properties Pot. Formate Lignite Lignite 600 RPM 50 46 42 300 RPM 40 33 33 200 RPM 37 30 30 100 RPM 32 28 29  6 RPM 9 9 14  3 RPM 5 8 10  10 sec Gel (Pa) 3 4 5 PV (mPa * s) 10 13 9 YP (Pa) 15 10 12 LSRV (cP) 30,000 29,000 31,000 Temperature 20.1 20.1 20.1 (° C.)

TABLE 13 Results using Calcium Nitrate Sample #1 + 5% Sample #1 + Sample #1 + Calcium Nitrate + 5% Calcium Nitrate + 5% Calcium 5 kg/m3 15 kg/m3 Mud Properties Nitrate Lignite Lignite 600 RPM 58 49 44 300 RPM 52 42 38 200 RPM 50 41 37 100 RPM 47 35 32  6 RPM 12 11 14  3 RPM 8 8 8  10 sec Gel (Pa) 5 4.5 4.5 PV (mPa * s) 6 7 6 YP (Pa) 23 17.5 16 LSRV (cP) 35,000 43,000 23,000 Temperature 20.1 20.1 20.1 (° C.) Note: Lignite dissolves slower.

TABLE 14 Results using Calcium Chloride Sample #1 + Sample #1 + 5% Calcium 5% Calcium Sample #1 + Chloride + Chloride + 5% Calcium 5 kg/m3 15 kg/m3 Mud Properties Chloride Lignite Lignite 600 RPM 63 48 43 300 RPM 50 37 34 200 RPM 42 34 31 100 RPM 35 29 29  6 RPM 13 12 13  3 RPM 10 9 11  10 sec Gel (Pa) 6.5 6.5 7 PV (mPa * s) 13 11 9 YP (Pa) 18.5 13 11.5 LSRV (cP) 40,000 37,000 27,000 Temperature (° C.) 20.1 20.1 20.1 Note: Lignite dissolves slower.

TABLE 15 Results using Potassium Sulfate Sample #1 + Sample #1 + Sample #1 + 5% Pot. Sulfate + 5% Pot. Sulfate + 5% 5 kg/m3 15 kg/m3 Mud Properties Pot. Sulfate Lignite Lignite 600 RPM 165 128 91 300 RPM 150 115 76 200 RPM 143 109 71 100 RPM 131 100 63  6 RPM 85 67 42  3 RPM 37 58 39  10 sec Gel (Pa) 16 29 22 PV (mPa * s) 15 13 15 YP (Pa) 77.5 51 30.5 LSRV (cP) 100,000+ 80,000 67,000 Temperature (° C.) 20.1 20.1 20.1

TABLE 16 Results using Sodium Sulfate Sample #1 + 2% Sample #1 + Sample #1 + Sodium Sulfate + 2% Sodium Sulfate + 2% Sodium 5 kg/m3 15 kg/m3 Mud Properties Sulfate Lignite Lignite 600 RPM 179 39 31 300 RPM 155 25 19 200 RPM 143 20 15 100 RPM 123 14 9  6 RPM 72 8 3  3 RPM 63 7 2  10 sec Gel (Pa) 31 5 2.5 PV (mPa * s) 24 14 13 YP (Pa) 65.5 5.5 4 LSRV (cP) 90,000 50,000 28,000 Temperature 22.0 22.0 22.0 (° C.)

TABLE 17 Results using Sodium Sulfate Sample #1 + 5% Sample #1 + Sample #1 + Sodium Sulfate + 5% Sodium Sulfate + 5% Sodium 5 kg/m3 15 kg/m3 Mud Properties Sulfate Lignite Lignite 600 RPM 207 48 33 300 RPM 174 38 22 200 RPM 152 35 18 100 RPM 124 31 13  6 RPM 74 27 11  3 RPM 67 26 10  10 sec Gel (Pa) 28 14 9 PV (mPa * s) 33 10 11 YP (Pa) 70.5 14 5.5 LSRV (cP) 100,000 100,000 80,000 Temperature 22.0 22.0 22.0 (° C.)

Example II

Background: Nr Wetaskiwin, Alberta, Drilled 222 mm hole to Intermediate Casing Depth of 1425 mMD and set casing at ˜86.2 degrees inclination in the Rex Coal formation. Set and cement 177.8 mm casing.

Drilling Fluid: 60 m3 of mud is premixed with the following formulation: 30 kg/m3 of natural bentonite is pre-hydrated in fresh water for 16 hours. 3 kg/m3 of PolyVis II (MMH) is added over 2 hours. pH is raised to 12.0 with caustic via chemical barrel over pre-mix tank. Fluid becomes viscous. 50 kg/m3 of Potassium Sulphate is added.

Drilling in Coal: Intermediate casing shoe and cement are drilled out with a 156 mm bit using water and then water is displaced over to the pre-mixed system, described above. This well was drilled horizontally in the Rex Coal formation using the pre-mixed system.

Fluid Properties prior to drilling coal:

Premix: 60 m3 circulating system.

Depth: 1425 m (87.2 degrees inclination)

Funnel Viscosity: 55 s/L

Mud density: 1050 kg/m3

pH: 12.0

600 reading: 64

300 reading: 61

200 reading: 60

100 reading: 56

6 reading: 36

3 reading: 23

PV (mPa·s): 3

YP (Pa): 29

Gels (Pa): 11/11

Filtrate (Fluid Loss, mls/30 min): no control

MBT: 30 Kg/m3

Potassium ion (mg/L): 25,000

Fluid properties after drilling to 1451 m in Rex Coal formation:

Depth: 1451 m (88 degrees inclination)

Funnel Viscosity: 66 s/L

Mud density: 1060 kg/m3

pH: 11.5

600 reading: 62

300 reading: 55

200 reading: —

100 reading: —

6 reading: —

3 reading: —

PV (mPa·s): 7

YP (Pa): 24

Gels (Pa): 6/10

Filtrate (Fluid Loss, mls/30 min): 60

MBT: 24 Kg/m3

Potassium ion (mg/L): 22,000

It was determined that the fluid viscosity remained substantially stable despite drilling pure coal.

Thereafter drilling continued to 1845 m in Rex Coal formation with the addition of 15×22.7 kg sacks of non-ionic starch (Unitrol Starch) for fluid loss control into 80 m3 system:

Fluid properties at depth 1845 m (91.4 degrees inclination):

Funnel Viscosity: 59 s/L

Mud density: 1050 kg/m3

pH: 12.0

600 reading: 64

300 reading: 56

200 reading: —

100 reading: —

6 reading: —

3 reading: —

PV (mPa·s): 8

YP (Pa): 24

Gels (Pa): 9/11

Filtrate (Fluid Loss, mls/30 min): 19

MBT: 22 Kg/m3

Potassium ion (mg/L): 20,400

The addition of starch doesn't affect the rheology substantially.

After drilling to 2050 m in the Rex Coal formation the fluid properties were as follows (89 m3 system):

Depth: 2050 m (87.8 degrees inclination)

Funnel Viscosity: 85 s/L

Mud density: 1050 kg/m3

pH: 12.0

600 reading: 80

300 reading: 70

200 reading: 65

100 reading: 60

6 reading: 47

3 reading: 44

PV (mPa·s): 10

YP (Pa): 30

Gels (Pa): 17/18

Filtrate (Fluid Loss, mls/30 min): 15

MBT: 25 Kg/m3

Potassium ion (mg/L): 22,500

It was determined that a mixed metal viscosified—natural bentonite type rheology can be maintained when drilling through coal with the present system.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to those embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims, wherein reference to an element in the singular, such as by use of the article “a” or “an” is not intended to mean “one and only one” unless specifically so stated, but rather “one or more”. All structural and functional equivalents to the elements of the various embodiments described throughout the disclosure that are know or later come to be known to those of ordinary skill in the art are intended to be encompassed by the elements of the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 USC 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or “step for”. 

We claim:
 1. A method for drilling a well into a formation, the method comprising: providing a mixed metal-viscosified drilling fluid including 15 to 45 kg/m³ bentonite, and a mixed metal viscosifier of inorganic magnesium/aluminum hydroxide and greater than 3% and up to 10% potassium sulfate (w/v); circulating the drilling fluid through the well; and maintaining the viscosity of the drilling fluid at a yield point of greater than 10 Pa.
 2. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes providing an aqueous-based drilling fluid including the mixed metal viscosifier at a weight ratio of 1:8 to 1:12, viscosifier to bentonite, a base to maintain the pH above about 10.0 and the potassium sulfate.
 3. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes providing an aqueous-based drilling fluid including about 30 kg/m³ bentonite clay, the mixed metal viscosifier in a quantity of about 1:10 mixed metal viscosifier to bentonite with a pH controlled to greater than pH 11 and the potassium sulfate to an upper limit of 5%.
 4. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes: mixing bentonite clay in water to form a bentonite mixture; adding the mixed metal viscosifier to the bentonite mixture; adjusting pH to greater than above about pH 10; adding the potassium sulfate.
 5. The method of claim 4 further comprising adding any of fluid loss control additives and/or lost circulation materials.
 6. The method of claim 1 further comprising continuing circulation with entrained coal fines in the drilling fluid.
 7. The method of claim 6 wherein rheology of the drilling fluid is substantially maintained.
 8. The method of claim 1 wherein circulating the drilling fluid is maintained while a coal seam is open to the drilling fluid.
 9. The method of claim 1 wherein circulating the drilling fluid is initiated substantially at surface.
 10. The method of claim 4 wherein the pH is adjusted using caustic soda. 